Plants adapt to various types of environmental factors of their roots. For example, plants have a capacity to change shapes corresponding to the environment of growth and development. Once a plant has sent down roots in a given place, if the environment of that place somehow becomes unsuitable to that plant, the plant cannot move, but adapts and continues to grow and develop by changing its own shape. The ability to change shapes is an essential condition of survival for a plant.
Cross-breeding, breeding using recent genetic engineering techniques, and methods utilizing the action of plant hormones and plant regulators have been implemented in order to improve various types of morphological modification.
Numerous studies have been conducted using Arabidopsis thaliana in order to understand the control mechanisms of the morphogenesis and morphological modification of plants. There have been reports of LEAFY (LFY), APETALA1 (AP1), AGAMOUS (AG), SUPERMAN (SUP) and FIL (FIL) as genes that participate in flower formation. There have been reports of TCH4 and dbp, etc. as genes that participate in stem formation. There have been reports of LAN1 and MAC as genes that participate in leaf formation. There have been reports of ttg, g12, CPC and IRE as genes that participate in root formation.
Plants with improved morphological modification and morphogenesis have already been created using genetic engineering techniques. There have been reports of morphological changes of the flowers of petunia by introducing a transcription factor (PetSPL3) having a zinc finger motif (Japanese Enexamined Patent Publication No. 262390/1999). There have been reports of morphological changes of the roots of Arabidopsis thaliana by introducing a CPC gene or an IRE gene (Japanese Enexamined Patent Publication No. 4978/1998, Japanese Enexamined Patent Publication No. 270873/2000). However, most plants in which these genes have been transgenetically modified have not actually obtained sufficient effect to have industrial applicability, and currently a practical level has not yet been reached.
Polyamines, the general term for aliphatic hydrocarbons with 2 or more primary amino groups, are ubiquitous natural substances in organisms, with more than 20 types discovered so far. Typical polyamines include putrescine, spermidine, and spermine. The known primary physiological action of polyamines includes (1) nucleic acid stabilization and structural modification through interaction with nucleic acids; (2) promotion of various nucleic acid synthesis systems; (3) activation of protein synthesis systems; and (4) stabilization of cell membranes and enhancement of membrane permeability of substances.
Reports on the role of polyamines in plants include cell protection and promotion of nucleic acid or protein biosynthesis during cellular growth or division. The involvement of polyamines in various types of environmental stress has recently attracted attention.
There have been several reports on morphological modification and polyamine relating to somatic embryogenesis and adventitious root formation. For example, it has been demonstrated that polyamine promotes somatic embryogenesis in carrots (Planta, 162, 532, 1984, Science, 223, 1433, 1984), oats (Plant Growth Reg., 3, 329, 1985), Chinese cabbage (Plant Sci., 56, 167, 1988), and petunia (Plant Sci., 62, 123, 1989), and promotes adventitious root formation in mung beans (Physiol. Plant., 64, 53, 1985, Plant Cell Physiol., 24, 677, 1983). Thus far, there have been hardly any reports on the participation of polyamine in the morphogenesis of flowers, stems, leaves, and ovaries.
Known polyamine metabolism-related enzymes involved in the biosynthesis of plant polyamines include arginine decarboxylase (ADC), ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (SAMDC), spermidine synthase (SPDS), and spermine synthase (SPMS). Several polyamine metabolism-related enzyme genes coding for such polyamine metabolism-related enzymes have already been isolated from plants. The ADC gene has been isolated from oats (Mol. Gen. Genet., 224, 431-436 (1990)), tomatoes (Plant Physiol., 103, 829-834 (1993)), Arabidopsis thaliana (Plant Physiol., 111, 1077-1083 (1996)), and peas (Plant Mol. Biol., 28, 997-1009 (1995)); the ODC gene has been isolated from datura (Biochem. J., 314, 241-248 (1996)); the SAMDC gene has been isolated from potatoes (Plant Mol. Biol., 26, 327-338 (1994)), spinach (Plant Physiol., 107, 1461-1462 (1995)), and tobacco; and the SPDS gene has been isolated from Arabidopsis thaliana (Plant Cell Physiol., 39(1), 73-79 (1998)).
Thus, an object of the present invention is to produce recombinant plants with various types of improved morphogenesis by artificially controlling the expression of polyamine metabolism-related enzyme genes, and by varying the polyamine levels.